Blacksburg, Va. – An unusual bowstring truss iron bridge
that carried traffic across Roaring Run in Bedford County, Va. for
almost 100 years is now a picturesque footbridge at the I-81
Ironto, Va. rest stop. Built in 1878, it is the oldest standing
metal bridge in Virginia. In early December, a Virginia Tech
undergraduate conducted a load-bearing analysis of the
structure.
It may have been the first such test on the bridge. "There was
no documentation of a structural analysis from when the bridge was
designed," said Elaine Huffman of Bowie, Md., a student in civil
engineering. As part of her research project, she did a historical
survey of the bridge through a literature review, developed a
computer-based structural analysis, and recently verified the
computer model with an actual load test.
The bowstring truss design was patented by Z. King in 1859 under
the name "Tubular Arch Bridge," Huffman learned. When the bridge
was bypassed by a pipe culvert in the 1970s, it began to fall into
disrepair. "The Virginia Transportation Research Council recognized
the importance of the bridge as a historical landmark and worked to
preserve it by restoring it and putting it into use elsewhere,"
said Huffman. "Much work was put into determining the original
paint scheme and recreating it once the bridge was relocated. The
new site was selected to maintain the function of the bridge."
Huffman noted a number of unique features of the wrought iron
bridge. For example, "there is a unique bracing system
perpendicular to the truss that restrains lateral movement of the
arch," she said. "Cross braces prevent longitudinal motion of the
bridge deck as it hangs from the vertical cables."
In her computer model stress analysis, Huffman applied three
different loads, two of them from the era of iron bridges. One test
came from the 1893 Practical Treatise on the Construction of Iron
Highway Bridges, which suggested that a uniform distributed load of
75 pounds per square foot (psf) be applied to ordinary country
bridges 60 feet and shorter to represent a typical load. For a
vehicular point load, the 1898 work, De Prontibus, suggested using
a six-foot by eight-foot wagon load of five tons distributed
equally between all four wheels. The third load was representative
of the three-ton truck that would be used in the load test.
The deflected shape of the truss with the 3.5-ton wagon load was
the same as the five-ton wagon load, Huffman determined. The
uniform load created the highest stresses and highest deflections.
"Generally, stresses are limited in a modern bridge design to 60
percent of the yield stress in service," Huffman said. "But in its
current location, the bridge will most likely never see such high
stresses because pedestrian traffic over it is neither constant nor
high enough."
On December 3, Huffman carried out a load test to verify the
accuracy of the computer model. Dial gages, which turn small linear
movements into readable increments on a dial, were set up below the
center of each truss. Then, a truck weighing three tons was driven
across the bridge, pausing every five feet to record the
deflection.
"The bridge behaved as expected for the most part. The maximum
deflection recorded for one truss was 0.14 inches, 70 percent of
the result predicted by the model. However, the second truss
deflected a smaller amount," Huffman said. "Preliminary analysis
suggests that the diagonal cable members have loosened over time
and are supporting the bridge loads unevenly, allowing one truss to
deflect more than the other,"
The results from this test will be contributed to the Adaptive
Bridge Use Project based at the University of Massachusetts Amherst
and supported by the National Science Foundation (www.ecs.umass.edu/adaptive_bridge_use/).
The program aims to restore and study historic bridges to enhance
structural engineering curriculum and preserve examples of bridge
designs from the past, said Huffman's advisor, Cris Moen, assistant
professor of civil and environmental engineering.
The Ironto Wayside footbridge, the last remaining bowstring
arch-truss in the state, is a significant landmark in Virginia. "It
is useful to study historic landmarks, as they can guide us in the
design of future structures," said Huffman.
Her computer model can be used as an example for creating
structural models to test other bridges, said Moen.
"Perhaps the analysis will aid in the future assessment of the
bridge's condition as it continues to be preserved as a historic
landmark," said Huffman.
SOURCE